THE MOON
What we call the moon's "phases" are merely the various ways in which we
see the sun shining upon her surface during the course of her monthly
revolutions around the earth (see Fig. 14, p. 184). When she passes in
the neighbourhood of the sun all his light falls upon that side which is
turned away from us, and so the side which is turned towards us is
unillumined, and therefore invisible. When in this position the moon is
spoken of as _new_.
As she continues her motion around the earth, she draws gradually to the
east of the sun's place in the sky. The sunlight then comes somewhat
from the side; and so we see a small portion of the right side of the
lunar disc illuminated. This is the phase known as the _crescent_ moon.
As she moves on in her orbit more and more of her illuminated surface is
brought into view; and so the crescent of light becomes broader and
broader, until we get what is called half-moon, or _first quarter_, when
we see exactly one-half of her surface lit up by the sun's rays. As she
draws still further round yet more of her illuminated surface is brought
into view, until three-quarters of the disc appear lighted up. She is
then said to be _gibbous_.
Eventually she moves round so that she faces the sun completely, and
the whole of her disc appears illuminated. She is then spoken of as
_full_. When in this position it is clear that she is on the contrary
side of the earth to the sun, and therefore rises about the same time
that he is setting. She is now, in fact, at her furthest from the sun.
[Illustration: Direction from which the sun's rays are coming.
Various positions and illumination of the mooon by the sun during her
revolution around the earth.
The corresponding positions as viewed from the earth, showing the
consequent phases.
FIG. 14.--Orbit and Phases of the Moon.]
After this, the motion of the moon in her orbit carries her on back
again in the direction of the sun. She thus goes through her phases as
before, only these of course are _in the reverse order_. The full phase
is seen to give place to the gibbous, and this in turn to the half-moon
and to the crescent; after which her motion carries her into the
neighbourhood of the sun, and she is once more new, and lost to our
sight in the solar glare. Following this she draws away to the east of
the sun again, and the old order of phases repeat themselves as before.
The early Babylonians imagined that the moon had a bright and a dark
side, and that her phases were caused by the bright side coming more and
more into view during her movement around the sky. The Greeks, notably
Aristotle, set to work to examine the question from a mathematical
standpoint, and came to the conclusion that the crescent and other
appearances were such as would necessarily result if the moon were a
dark body of spherical shape illumined merely by the light of the sun.
Although the true explanation of the moon's phases has thus been known
for centuries, it is unfortunately not unusual to see
pictures--advertisement posters, for instance--in which stars appear
_within_ the horns of a crescent moon! Can it be that there are to-day
educated persons who believe that the moon is a thing which _grows_ to a
certain size and then wastes away again; who, in fact, do not know that
the entire body of the moon is there all the while?
When the moon shows a very thin crescent, we are able dimly to see her
still dark portion standing out against the sky. This appearance is
popularly known as the "old moon in the new moon's arms." The dark part
of her surface must, indeed, be to some degree illumined, or we should
not be able to see it at all. Whence then comes the light which
illumines it, since it clearly cannot come from the sun? The riddle is
easily solved, if we consider what kind of view of our earth an observer
situated on this darkened part of the moon would at that moment get. He
would, as a matter of fact, just then see nearly the whole disc of the
earth brightly lit up by sunlight. The lunar landscape all around would,
therefore, be bathed in what to _him_ would be "earthlight," which of
course takes the place there of what _we_ call moonlight. If, then, we
recollect how much greater in size the earth is than the moon, it should
not surprise us that this earthlight will be many times brighter than
moonlight. It is considered, indeed, to be some twenty times brighter.
It is thus not at all astonishing that we can see the dark portion of
the moon illumined merely by sunlight reflected upon it from our earth.
The ancients were greatly exercised in their minds to account for this
"earthlight," or "earthshine," as it is also called. Posidonius (135-51
B.C.) tried to explain it by supposing that the moon was partially
transparent, and that some sunlight consequently filtered through from
the other side. It was not, however, until the fifteenth century that
the correct solution was arrived at.
[Illustration: One side of the moon only is ever presented to the
earth. This side is here indicated by the letters S.F.E. (side facing
earth).
By placing the above positions in a row, we can see at once that the
moon makes one complete rotation on her axis in exactly the same time as
she revolves around the earth.
FIG. 15.--The Rotation of the Moon on her Axis.]
Perhaps the most remarkable thing which one notices about the moon is
that she always turns the same side towards us, and so we never see her
other side. One might be led from this to jump to the conclusion that
she does not rotate upon an axis, as do the other bodies which we see;
but, paradoxical as it may appear, the fact that she turns one face
always towards the earth, is actually a proof that she _does_ rotate
upon an axis. The rotation, however, takes place with such slowness,
that she turns round but once during the time in which she revolves
around the earth (see Fig. 15). In order to understand the matter
clearly, let the reader place an object in the centre of a room and walk
around it once, _keeping his face turned towards it the whole time_,
While he is doing this, it is evident that he will face every one of the
four walls of the room in succession. Now in order to face each of the
four walls of a room in succession one would be obliged _to turn oneself
entirely round_. Therefore, during the act of walking round an object
with his face turned directly towards it, a person at the same time
turns his body once entirely round.
In the long, long past the moon must have turned round much faster than
this. Her rate of rotation has no doubt been slowed down by the action
of some force. It will be recollected how, in the course of the previous
chapter, we found that the tides were tending, though exceedingly
gradually, to slow down the rotation of the earth upon its axis. But, on
account of the earth's much greater mass, the force of gravitation
exercised by it upon the surface of the moon is, of course, much more
powerful than that which the moon exercises upon the surface of the
earth. The tendency to tidal action on the moon itself must, therefore,
be much in excess of anything which we here experience. It is, in
consequence, probable that such a tidal drag, extending over a very long
period of time, has resulted in slowing down the moon's rotation to its
present rate.
The fact that we never see but one side of the moon has given rise from
time to time to fantastic speculations with regard to the other side.
Some, indeed, have wished to imagine that our satellite is shaped like
an egg, the more pointed end being directed away from us. We are here,
of course, faced with a riddle, which is all the more tantalising from
its appearing for ever insoluble to men, chained as they are to the
earth. However, it seems going too far to suppose that any abnormal
conditions necessarily exist at the other side of the moon. As a matter
of fact, indeed, small portions of that side are brought into our view
from time to time in consequence of slight irregularities in the moon's
movement; and these portions differ in no way from those which we
ordinarily see. On the whole, we obtain a view of about 60 per cent. of
the entire lunar surface; that is to say, a good deal more than
one-half.
The actual diameter of the moon is about 2163 miles, which is somewhat
more than one-quarter the diameter of the earth. For a satellite,
therefore, she seems very large compared with her primary, the earth;
when we consider that Jupiter's greatest satellite, although nearly
twice as broad as our moon, has a diameter only one twenty-fifth that of
Jupiter. Furthermore, the moon moves around the earth comparatively
slowly, making only about thirteen revolutions during the entire year.
Seen from space, therefore, she would not give the impression of a
circling body, as other satellites do. Her revolutions are, indeed,
relatively so very slow that she would appear rather like a smaller
planet accompanying the earth in its orbit. In view of all this, some
astronomers are inclined to regard the earth and moon rather as a
"double planet" than as a system of planet and satellite.
When the moon is full she attracts more attention perhaps than in any of
her other phases. The moon, in order to be full, must needs be in that
region of the heavens exactly opposite to the sun. The sun _appears_ to
go once entirely round the sky in the course of a year, and the moon
performs the same journey in the space of about a month. The moon, when
full, having got half-way round this journey, occupies, therefore, that
region of the sky which the sun itself will occupy half a year later.
Thus in winter the full moon will be found roughly to occupy the sun's
summer position in the sky, and in summer the sun's winter position. It
therefore follows that the full moon in winter time is high up in the
heavens, while in summer time it is low down. We thus get the greatest
amount of full moonlight when it is the most needed.
The great French astronomer, Laplace, being struck by the fact that the
"lesser light" did not rule the night to anything like the same extent
that the "greater light" ruled the day, set to work to examine the
conditions under which it might have been made to do so. The result of
his speculations showed that if the moon were removed to such a distance
that she took a year instead of a month to revolve around the earth; and
if she were started off in her orbit at full moon, she would always
continue to remain full--a great advantage for us. Whewell, however,
pointed out that in order to get the moon to move with the requisite
degree of slowness, she would have to revolve so far from the earth that
she would only look one-sixteenth as large as she does at present, which
rather militates against the advantage Laplace had in mind! Finally,
however, it was shown by M. Liouville, in 1845, that the position of a
_perennial full moon_, such as Laplace dreamed of, would be
unstable--that is to say, the body in question could not for long remain
undisturbed in the situation suggested (see Fig. 16, p. 191).
[Illustration: Various positions of Laplace's "Moon" with regard to the
earth and sun during the course of a year.
The same positions of Laplace's "Moon," arranged around the earth, show
that it would make only one revolution in a year.
FIG. 16.--Laplace's "Perennial Full Moon."]
There is a well-known phenomenon called _harvest moon_, concerning the
nature of which there seems to be much popular confusion. An idea in
fact appears to prevail among a good many people that the moon is a
harvest moon when, at rising, it looks bigger and redder than usual.
Such an appearance has, however, nothing at all to say to the matter;
for the moon always _looks_ larger when low down in the sky, and,
furthermore, it usually looks red in the later months of the year, when
there is more mist and fog about than there is in summer. What
astronomers actually term the harvest moon is, indeed, something
entirely different from this. About the month of September the slant at
which the full moon comes up from below the horizon happens to be such
that, during several evenings together, she _rises almost at the same
hour_, instead of some fifty minutes later, as is usually the case. As
the harvest is being gathered in about that time, it has come to be
popularly considered that this is a provision of nature, according to
which the sunlight is, during several evenings, replaced without delay
by more or less full-moonlight, in order that harvesters may continue
their work straight on into the night, and not be obliged to break off
after sunset to wait until the moon rises. The same phenomenon is almost
exactly repeated a month later, but by reason of the pursuits then
carried on it is known as the "hunter's moon."
In this connection should be mentioned that curious phenomenon above
alluded to, and which seems to attract universal notice, namely, that
the moon _looks much larger when near the horizon_--at its rising, for
instance, than when higher up in the sky. This seeming enlargement is,
however, by no means confined to the moon. That the sun also looks much
larger when low down in the sky than when high up, seems to strike even
the most casual watcher of a sunset. The same kind of effect will,
indeed, be noted if close attention be paid to the stars themselves. A
constellation, for instance, appears more spread out when low down in
the sky than when high up. This enlargement of celestial objects when in
the neighbourhood of the horizon is, however, only _apparent_ and not
real. It must be entirely an _illusion_; for the most careful
measurements of the discs of the sun and of the moon fail to show that
the bodies are any larger when near the horizon than when high up in the
sky. In fact, if there be any difference in measurements with regard to
the moon, it will be found to be the other way round; for her disc, when
carefully measured, is actually the slightest degree _greater_ when
_high_ in the sky, than when low down. The reason for this is that, on
account of the rotundity of the earth's surface, she is a trifle nearer
the observer when overhead of him.
This apparent enlargement of celestial objects, when low down in the
sky, is granted on all sides to be an illusion; but although the
question has been discussed with animation time out of mind, none of the
explanations proposed can be said to have received unreserved
acceptance. The one which usually figures in text-books is that we
unconsciously compare the sun and moon, when low down in the sky, with
the terrestrial objects in the same field of view, and are therefore
inclined to exaggerate the size of these orbs. Some persons, on the
other hand, imagine the illusion to have its source in the structure of
the human eye; while others, again, put it down to the atmosphere,
maintaining that the celestial objects in question _loom_ large in the
thickened air near the horizon, in the same way that they do when viewed
through fog or mist.
The writer[14] ventures, however, to think that the illusion has its
origin in our notion of the shape of the celestial vault. One would be
inclined, indeed, to suppose that this vault ought to appear to us as
the half of a hollow sphere; but he maintains that it does not so
appear, as a consequence of the manner in which the eyes of men are set
quite close together in their heads. If one looks, for instance, high up
in the sky, the horizon cannot come within the field of view, and so
there is nothing to make one think that the expanse then gazed upon is
other than quite _flat_--let us say like the ceiling of a room. But, as
the eyes are lowered, a portion of the _encircling_ horizon comes
gradually into the field of view, and the region of the sky then gazed
upon tends in consequence to assume a _hollowed-out_ form. From this it
would seem that our idea of the shape of the celestial vault is, that it
is _flattened down over our heads and hollowed out all around in the
neighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as a
consequence of their very great distance, all the objects in the heavens
necessarily appear to us to move as if they were placed on the
background of the vault; the result being that the mind is obliged to
conceive them as expanded or contracted, in its unconscious attempts to
make them always fill their due proportion of space in the various parts
of this abnormally shaped sky.
From such considerations the writer concludes that the apparent
enlargement in question is merely the natural consequence of the idea we
have of the shape of the celestial vault--an idea gradually built up in
childhood, to become later on what is called "second nature." And in
support of this contention, he would point to the fact that the
enlargement is not by any means confined to the sun and moon, but is
every whit as marked in the case of the constellations. To one who has
not noticed this before, it is really quite a revelation to compare the
appearance of one of the large constellations (Orion, for instance) when
high up in the sky and when low down. The widening apart of the various
stars composing the group, when in the latter position, is very
noticeable indeed.
[Illustration: FIG. 17.--Illustrating the author's explanation of the
apparent enlargement of celestial objects.]
Further, if a person were to stand in the centre of a large dome, he
would be exactly situated as if he were beneath the vaulted heaven, and
one would consequently expect him to suffer the same illusion as to the
shape of the dome. Objects fixed upon its background would therefore
appear to him under the same conditions as objects in the sky, and the
illusions as to their apparent enlargement should hold good here also.
Some years ago a Belgian astronomer, M. Stroobant, in an investigation
of the matter at issue, chanced to make a series of experiments under
the very conditions just detailed. To various portions of the inner
surface of a large dome he attached pairs of electric lights; and on
placing himself at the centre of the building, he noticed that, in every
case, those pairs which were high up appeared closer together than those
which were low down! He does not, however, seem to have sought for the
cause in the vaulted expanse. On the contrary, he attributed the effect
to something connected with our upright stature, to some physiological
reason which regularly makes us estimate objects as larger when in front
than when overhead.
In connection with this matter, it may be noted that it always appears
extremely difficult to estimate with the eye the exact height above the
horizon at which any object (say a star) happens to be. Even skilled
observers find themselves in error in attempting to do so. This seems to
bear out the writer's contention that the form under which the celestial
vault really appears to us is a peculiar one, and tends to give rise to
false judgments.
Before leaving this question, it should also be mentioned that nothing
perhaps is more deceptive than the size which objects in the sky appear
to present. The full moon looks so like a huge plate, that it astonishes
one to find that a threepenny bit held at arm's length will a long way
more than cover its disc.
[Illustration: PLATE VIII. THE MOON
From a photograph taken at the Paris Observatory by M.P. Puiseux.
(Page 197)]
The moon is just too far off to allow us to see the actual detail on
her surface with the naked eye. When thus viewed she merely displays a
patchy appearance,[15] and the imaginary forms which her darker markings
suggest to the fancy are popularly expressed by the term "Man in the
Moon." An examination of her surface with very moderate optical aid is,
however, quite a revelation, and the view we then get is not easily
comparable to what we see with the unaided eye.
Even with an ordinary opera-glass, an observer will be able to note a
good deal of detail upon the lunar disc. If it be his first observation
of the kind, he cannot fail to be struck by the fact to which we have
just made allusion, namely, the great change which the moon appears to
undergo when viewed with magnifying power. "Cain and his Dog," the "Man
in the Moon gathering sticks," or whatever indeed his fancy was wont to
conjure up from the lights and shades upon the shining surface, have now
completely disappeared; and he sees instead a silvery globe marked here
and there with extensive dark areas, and pitted all over with
crater-like formations (see Plate VIII., p. 196). The dark areas retain
even to the present day their ancient name of "seas," for Galileo and
the early telescopic observers believed them to be such, and they are
still catalogued under the mystic appellations given to them in the long
ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of
Dreams."[16] The improved telescopes of later times showed, however,
that they were not really seas (there is no water on the moon), but
merely areas of darker material.
The crater-like formations above alluded to are the "lunar mountains." A
person examining the moon for the first time with telescopic aid, will
perhaps not at once grasp the fact that his view of lunar mountains must
needs be what is called a "bird's-eye" one, namely, a view from above,
like that from a balloon and that he cannot, of course, expect to see
them from the side, as he does the mountains upon the earth. But once he
has realised this novel point of view, he will no doubt marvel at the
formations which lie scattered as it were at his feet. The type of lunar
mountain is indeed in striking contrast to the terrestrial type. On our
earth the range-formation is supreme; on the moon the crater-formation
is the rule, and is so-called from analogy to our volcanoes. A typical
lunar crater may be described as a circular wall, enclosing a central
plain, or "floor," which is often much depressed below the level of the
surface outside. These so-called "craters," or "ring-mountains," as they
are also termed, are often of gigantic proportions. For instance, the
central plain of one of them, known as Ptolemæus,[17] is about 115 miles
across, while that of Plato is about 60. The walls of craters often rise
to great heights; which, in proportion to the small size of the moon,
are very much in excess of our highest terrestrial elevations.
Nevertheless, a person posted at the centre of one of the larger craters
might be surprised to find that he could not see the encompassing
crater-walls, which would in every direction be below his horizon. This
would arise not alone from the great breadth of the crater itself, but
also from the fact that the curving of the moon's surface is very sharp
compared with that of our earth.
[Illustration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINCIPAL
"CRATERS," MOUNTAIN RANGES, AND "SEAS"
In this, as in the other plates of the Moon, the _South_ will be found
at the top of the picture; such being the view given by the ordinary
astronomical telescope, in which all objects are seen _inverted_.
(Page 199)]
We have mentioned Ptolemæus as among the very large craters, or
ring-mountains, on the moon. Its encompassing walls rise to nearly
13,000 feet, and it has the further distinction of being almost in the
centre of the lunar disc. There are, however, several others much wider,
but they are by no means in such a conspicuous position. For instance,
Schickard, close to the south-eastern border, is nearly 130 miles in
diameter, and its wall rises in one point to over 10,000 feet. Grimaldi,
almost exactly at the east point, is nearly as large as Schickard.
Another crater, Clavius, situated near the south point, is about 140
miles across; while its neighbour Bailly--named after a famous French
astronomer of the eighteenth century--is 180, and the largest of those
which we can see (see Plate IX., p. 198).
Many of the lunar craters encroach upon one another; in fact there is
not really room for them all upon the visible hemisphere of the moon.
About 30,000 have been mapped; but this is only a small portion, for
according to the American astronomer, Professor W.H. Pickering, there
are more than 200,000 in all.
Notwithstanding the fact that the crater is the type of mountain
associated in the mind with the moon, it must not be imagined that upon
our satellite there are no mountains at all of the terrestrial type.
There are indeed many isolated peaks, but strangely enough they are
nearly always to be found in the centres of craters. Some of these peaks
are of great altitude, that in the centre of the crater Copernicus being
over 11,000 feet high. A few mountain ranges also exist; the best known
of which are styled, the Lunar Alps and Lunar Apennines (see Plate X.,
p. 200).
Since the _mass_ of the moon is only about one-eightieth that of the
earth, it will be understood that the force of gravity which she
exercises is much less. It is calculated that, at her surface, this is
only about one-sixth of what we experience. A man transported to the
moon would thus be able to jump _six times as high_ as he can here. A
building could therefore be six times as tall as upon our earth, without
causing any more strain upon its foundations. It should not, then, be
any subject for wonder, that the highest peaks in the Lunar Apennines
attain to such heights as 22,000 feet. Such a height, upon a
comparatively small body like the moon, for her _volume_ is only
one-fiftieth that of the earth, is relatively very much in excess of the
29,000 feet of Himalayan structure, Mount Everest, the boast of our
planet, 8000 miles across!
High as are the Lunar Apennines, the highest peaks on the moon are yet
not found among them. There is, for instance, on the extreme southern
edge of the lunar disc, a range known as the Leibnitz Mountains; several
peaks of which rise to a height of nearly 30,000 feet, one peak in
particular being said to attain to 36,000 feet (see Plate IX., p. 198).
[Illustration: PLATE X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON
We have here (see "Map," Plate IX., p. 198) the mountain ranges of the
Apennines, the Caucasus and the Alps; also the craters Plato, Aristotle,
Eudoxus, Cassini, Aristillus, Autolycus, Archimedes and Linné. The
crater Linné is the very bright spot in the dark area at the upper left
hand side of the picture. From a photograph taken at the Paris
Observatory by M.M. Loewy and Puiseux.
(Page 200)]
But the reader will surely ask the question: "How is it possible to
determine the actual height of a lunar mountain, if one cannot go upon
the moon to measure it?" The answer is, that we can calculate its height
from noting the length of the shadow which it casts. Any one will allow
that the length of a shadow cast by the sun depends upon two things:
firstly, upon the height of the object which causes the shadow, and
secondly, upon the elevation of the sun at the moment in the sky. The
most casual observer of nature upon our earth can scarcely have failed
to notice that shadows are shortest at noonday, when the sun is at its
highest in the sky; and that they lengthen out as the sun declines
towards its setting. Here, then, we have the clue. To ascertain,
therefore, the height of a lunar mountain, we have first to consider at
what elevation the sun is at that moment above the horizon of the place
where the mountain in question is situated. Then, having measured the
actual length in miles of the shadow extended before us, all that is
left is to ask ourselves the question: "What height must an object be
whose shadow cast by the sun, when at that elevation in the sky, will
extend to this length?"
There is no trace whatever of water upon the moon. The opinion, indeed,
which seems generally held, is that water has never existed upon its
surface. Erosions, sedimentary deposits, and all those marks which point
to a former occupation by water are notably absent.
Similarly there appears to be no atmosphere on the moon; or, at any
rate, such an excessively rare one, as to be quite inappreciable. Of
this there are several proofs. For instance, in a solar eclipse the
moon's disc always stands out quite clear-cut against that of the sun.
Again during occultations, stars disappear behind the moon with a
suddenness, which could not be the case were there any appreciable
atmosphere. Lastly, we see no traces of twilight upon the lunar surface,
nor any softening at the edges of shadows; both which effects would be
apparent if there were an atmosphere.
The moon's surface is rough and rocky, and displays no marks of the
"weathering" that would necessarily follow, had it possessed anything of
an atmosphere in the past. This makes us rather inclined to doubt that
it ever had one at all. Supposing, however, that it did possess an
atmosphere in the past, it is interesting to inquire what may have
become of it. In the first place it might have gradually disappeared, in
consequence of the gases which composed it uniting chemically with the
materials of which the lunar body is constructed; or, again, its
constituent gases may have escaped into space, in accordance with the
principles of that kinetic theory of which we have already spoken. The
latter solution seems, indeed, the most reasonable of the two, for the
force of gravity at the lunar surface appears too weak to hold down any
known gases. This argument seems also to dispose of the question of
absence of water; for Dr. George Johnstone Stoney, in a careful
investigation of the subject, has shown that the liquid in question,
when in the form of vapour, will escape from a planet if its mass is
less than _one-fourth_ that of our earth. And the mass of the moon is
very much less than this; indeed only the _one-eightieth_, as we have
already stated.
In consequence of this lack of atmosphere, the condition of things upon
the moon will be in marked contrast to what we experience upon the
earth. The atmosphere here performs a double service in shielding us
from the direct rays of the sun, and in bottling the heat as a
glass-house does. On the moon, however, the sun beats down in the
day-time with a merciless force; but its rays are reflected away from
the surface as quickly as they are received, and so the cold of the
lunar night is excessive. It has been calculated that the day
temperature on the moon may, indeed, be as high as our boiling-point,
while the night temperature may be more than twice as low as the
greatest cold known in our arctic regions.
That a certain amount of solar heat is reflected to us from the moon is
shown by the sharp drop in temperature which certain heat-measuring
instruments record when the moon becomes obscured in a lunar eclipse.
The solar heat which is thus reflected to us by the moon is, however, on
the whole extremely small; more light and heat, indeed, reach us
_direct_ from the sun in half a minute than we get by _reflection_ from
the moon during the entire course of the year.
With regard to the origin of the lunar craters there has been much
discussion. Some have considered them to be evidence of violent volcanic
action in the dim past; others, again, as the result of the impact of
meteorites upon the lunar surface, when the moon was still in a plastic
condition; while a third theory holds that they were formed by the
bursting of huge bubbles during the escape into space of gases from the
interior. The question is, indeed, a very difficult one. Though
volcanic action, such as would result in craters of the size of
Ptolemæus, is hard for us to picture, and though the lone peaks which
adorn the centres of many craters have nothing reminiscent of them in
our terrestrial volcanoes, nevertheless the volcanic theory seems to
receive more favour than the others.
In addition to the craters there are two more features which demand
notice, namely, what are known as _rays_ and _rills_. The rays are long,
light-coloured streaks which radiate from several of the large craters,
and extend to a distance of some hundreds of miles. That they are mere
markings on the surface is proved by the fact that they cast no shadows
of any kind. One theory is, that they were originally great cracks which
have been filled with lighter coloured material, welling up from
beneath. The rills, on the other hand, are actually fissures, about a
mile or so in width and about a quarter of a mile in depth.
The rays are seen to the best advantage in connection with the craters
Tycho and Copernicus (see Plate XI., p. 204). In consequence of its
fairly forward position on the lunar disc, and of the remarkable system
of rays which issue from it like spokes from the axle of a wheel, Tycho
commands especial attention. The late Rev. T.W. Webb, a famous observer,
christened it, very happily, the "metropolitan crater of the moon."
[Illustration: PLATE XI. THE MOON
The systems of rays from the craters Tycho, Copernicus and Kepler are
well shown here. From a photograph taken at the Paris Observatory by
M.P. Puiseux.
(Page 204)]
A great deal of attention is, and has been, paid by certain astronomers
to the moon, in the hope of finding out if any changes are actually in
progress at present upon her surface. Sir William Herschel, indeed, once
thought that he saw a lunar volcano in eruption, but this proved to be
merely the effect of the sunlight striking the top of the crater
Aristarchus, while the region around it was still in shadow--sunrise
upon Aristarchus, in fact! No change of any real importance has,
however, been noted, although it is suspected that some minor
alterations have from time to time taken place. For instance, slight
variations of tint have been noticed in certain areas of the lunar
surface. Professor W.H. Pickering puts forward the conjecture that these
may be caused by the growth and decay of some low form of vegetation,
brought into existence by vapours of water, or carbonic acid gas, making
their way out from the interior through cracks near at hand.
Again, during the last hundred years one small crater known as Linné
(Linnæus), situated in the Mare Serenitatis (Sea of Serenity), has
appeared to undergo slight changes, and is even said to have been
invisible for a while (see Plate X., p. 200). It is, however, believed
that the changes in question may be due to the varying angles at which
the sunlight falls upon the crater; for it is an understood fact that
the irregularities of the moon's motion give us views of her surface
which always differ slightly.
The suggestion has more than once been put forward that the surface of
the moon is covered with a thick layer of ice. This is generally
considered improbable, and consequently the idea has received very
little support. It first originated with the late Mr. S.E. Peal, an
English observer of the moon, and has recently been resuscitated by the
German observer, Herr Fauth.
The most unfavourable time for telescopic study of the moon is when she
is full. The sunlight is then falling directly upon her visible
hemisphere, and so the mountains cast no shadows. We thus do not get
that impression of hill and hollow which is so very noticeable in the
other phases.
The first map of the moon was constructed by Galileo. Tobias Mayer
published another in 1775; while during the nineteenth century greatly
improved ones were made by Beer and Mädler, Schmidt, Neison and others.
In 1903, Professor W.H. Pickering brought out a complete photographic
lunar atlas; and a similar publication has recently appeared, the work
of MM. Loewy and Puiseux of the Observatory of Paris.
The so-called "seas" of the moon are, as we have seen, merely dark
areas, and there appears to be no proof that they were ever occupied by
any liquid. They are for the most part found in the _northern_ portion
of the moon; a striking contrast to our seas and oceans, which take up
so much of the _southern_ hemisphere of the earth.
There are many erroneous ideas popularly held with regard to certain
influences which the moon is supposed to exercise upon the earth. For
instance, a change in the weather is widely believed to depend upon a
change in the moon. But the word "change" as here used is meaningless,
for the moon is continually changing her phase during the whole of her
monthly round. Besides, the moon is visible over a great portion of the
earth _at the same moment_, and certainly all the places from which it
can then be seen do not get the same weather! Further, careful
observations, and records extending over the past one hundred years and
more, fail to show any reliable connection between the phases of the
moon and the condition of the weather.
It has been stated, on very good authority, that no telescope ever shows
the surface of the moon as clearly as we could see it with the naked eye
were it only 240 miles distant from us.
Supposing, then, that we were able to approach our satellite, and view
it without optical aid at such comparatively close quarters, it is
interesting to consider what would be the smallest detail which our eye
could take in. The question of the limit of what can be appreciated with
the naked eye is somewhat uncertain, but it appears safe to say that at
a distance of 240 miles the _minutest speck_ visible would have to be
_at least_ some 60 yards across.
Atmosphere and liquid both wanting, the lunar surface must be the seat
of an eternal calm; where no sound breaks the stillness and where
change, as we know it, does not exist. The sun beats down upon the arid
rocks, and inky shadows lie athwart the valleys. There is no mellowing
of the harsh contrasts.
We cannot indeed absolutely affirm that Life has no place at all upon
this airless and waterless globe, since we know not under what strange
conditions it may manifest its presence; and our most powerful
telescopes, besides, do not bring the lunar surface sufficiently near to
us to disprove the existence there of even such large creatures as
disport themselves upon our planet. Still, we find it hard to rid
ourselves of the feeling that we are in the presence of a dead world. On
she swings around the earth month after month, with one face ever
turned towards us, leaving a certain mystery to hang around that hidden
side, the greater part of which men can never hope to see. The rotation
of the moon upon her axis--the lunar day--has become, as we have seen,
equal to her revolution around the earth. An epoch may likewise
eventually be reached in the history of our own planet, when the length
of the terrestrial day has been so slowed down by tidal friction that it
will be equal to the year. Then will the earth revolve around the
central orb, with one side plunged in eternal night and the other in
eternal sunshine. But such a vista need not immediately distress us. It
is millions of years forward in time.
[14] _Journal of the British Astronomical Association_, vol. x.
(1899-1900), Nos. 1 and 3.
[15] Certain of the ancient Greeks thought the markings on the moon to
be merely the reflection of the seas and lands of our earth, as in a
badly polished mirror.
[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum.
[17] The lunar craters have, as a rule, received their names from
celebrated persons, usually men of science. This system of nomenclature
was originated by Riccioli, in 1651.